Go to the Options/Preferences page, Advanced section, Certificates panel, then click Certificate Manager. Make sure the Authorities tab is selected, then click Import. Find the file you downloaded and open it, then you should get a dialog - don't check any of the boxes, just click OK. Then you should see the new cert in the list in the Certificate Manager, as shown in Screenshot #1.

(2) Clear the time Firefox last re-verified

This will set Firefox up to check much sooner than it otherwise would check.

(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(B) In the search box above the list, type or paste xpi- and pause while the list is filtered

(C) Right-click the app.update.lastUpdateTime.xpi-signature-verification preference and click Reset on the context menu

Screenshot #2 illustrates the expected result.

(3) Restart Firefox

You can exit Firefox normally, then start it up again. It should take a few seconds, but then your addons should turn back into an active state

Go to the Options/Preferences page, Advanced section, Certificates panel, then click Certificate Manager. Make sure the Authorities tab is selected, then click Import. Find the file you downloaded and open it, then you should get a dialog - don't check any of the boxes, just click OK. Then you should see the new cert in the list in the Certificate Manager, as shown in Screenshot #1.

(2) Clear the time Firefox last re-verified

This will set Firefox up to check much sooner than it otherwise would check.

(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(B) In the search box above the list, type or paste xpi- and pause while the list is filtered

(C) Right-click the app.update.lastUpdateTime.xpi-signature-verification preference and click Reset on the context menu

Screenshot #2 illustrates the expected result.

(3) Restart Firefox

You can exit Firefox normally, then start it up again. It should take a few seconds, but then your addons should turn back into an active state

Hopefully that will work for you.

hi, sorry for this mishap! mozilla is considering in which way the problem should be addressed for users on legacy versions at the moment.
this would be an unofficial workaround i can offer in the meantime (basically the same steps that the automated hotfix was performing in the still supported versions of the browser):
===
(1) Download the fixed certificate
The updated necessary certificate is available for download at
https://hg.mozilla.org/releases/mozilla-release/raw-file/tip/security/apps/addons-public-intermediate.crt
Go to the Options/Preferences page, Advanced section, Certificates panel, then click Certificate Manager. Make sure the Authorities tab is selected, then click Import. Find the file you downloaded and open it, then you should get a dialog - don't check any of the boxes, just click OK. Then you should see the new cert in the list in the Certificate Manager, as shown in Screenshot #1.
(2) Clear the time Firefox last re-verified
This will set Firefox up to check much sooner than it otherwise would check.
(A) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.
(B) In the search box above the list, type or paste xpi- and pause while the list is filtered
(C) Right-click the '''app.update.lastUpdateTime.xpi-signature-verification''' preference and click Reset on the context menu
Screenshot #2 illustrates the expected result.
(3) Restart Firefox
You can exit Firefox normally, then start it up again. It should take a few seconds, but then your addons should turn back into an active state
Hopefully that will work for you.

There is a Legacy system extension coming or is already available to fix this issue for Firefox 52-60.
See comment 52.
*[https://bugzilla.mozilla.org/show_bug.cgi?id=1549604#c52 Bug 1549604] - ship legacy system add-ons to remediate armagadd-on 2.0

There are still unresolved issues that prevent installing the legacy hotfix automatically, but a manual install should work.

See comment 51 in bug 549604 for a link to install the legacy version of the hotfix extension to install the intermediate certificate.
*hotfix-update-xpi-signing-intermediate-bug-1548973-legacy@mozilla.org-1.1.2-signed.xpi
*https://bugzilla.mozilla.org/show_bug.cgi?id=1549604#c51
There are still unresolved issues that prevent installing the legacy hotfix automatically, but a manual install should work.